The present invention relates to the field of microelectromechanical systems and, more particularly, to microelectromechanical or micromechanical devices that actuate a moving element between operative positions to provide, for example, a switching operation.
A microelectromechanical system (MEMS) is a micro-device that is generally manufactured using integrated circuit fabrication or other similar techniques and therefore has the potential for cost-effective, large-scale production. A MEMS device is a high precision system used to sense, control, or actuate on very small scales by combining mechanical, electrical, magnetic, thermal and/or other physical phenomena. It typically includes a tiny mechanical device element such as a sensor, mirror, valve, or gear that is embedded in or deposited on a semiconductor chip or substrate. These systems may function individually, or they may be combined in array configurations to generate effects on a larger scale. Advantageously, a MEMS device may be monolithically integrated with driving, control, and/or signal processing microelectronics to improve performance and further reduce the cost of manufacturing, packaging, and instrumenting the device. As used herein, the term microelectromechanical (MEMS) device is intended to embrace devices that are physically small and have at least one component produced using micromachining or other microfabrication techniques, and the term MEMS device includes microactuators, micromechanical devices, and micromachine devices.
Due to their considerable technological potential, the use of MEMS is currently being pursued in many different fields. In particular, high precision MEMS are receiving an increasing amount of interest in the fiber-optics field because of their capability to overcome several limitations associated with prior art technologies: see generally Motamedi et al., xe2x80x9cMicro-opto-electro-mechanical devices and on-chip optical processingxe2x80x9d, Optical Engineering, vol. 36, no. 5, p. 1282 (May 1997), the contents of which are incorporated herein by virtue of this reference.
In fiber-optic communication systems, information is transmitted as a light or laser beam along a glass or plastic wire, known as a fiber. A significant amount of electronic communication and information transfer is effected through fiber-optic lines due to their much broader bandwidth and lower susceptibility to electromagnetic interference compared to conventional copper or metal wires. For example, much of the Internet and many long distance telephone communication networks are connected with fiber-optic lines. However, fast and efficient switching between optical fibers in a fiber-optic network has been difficult to achieve. Switches are needed to route signals at the backbone and gateway levels of these networks where one network connects with another, as well as at the sub-network level where data is being transported from its origin or to its destination. In addition, in a wavelength division multiplexed (WDM) optical fiber network, many channels, each occupying a distinct wavelength of light, may share the same fiber. In a WDM network, optical add-drop multiplexers and demultiplexers are used to introduce supplementary optical channels into the main optical fiber path and/or divert optical channels from the main fiber path.
Various prior art optical switching technologies have been employed. In electrical cross connect switch technology, the optical signal is transformed into an electrical signal, a switching operation is performed with an electronic switch, and the electrical signal is then transformed back into the optical domain. However, electrical cross connects are inefficient and costly. Another prior art solution is to use an optical switch or cross-connect (OXC) capable of connecting and disconnecting optical fibers in the optical domain. Integrated optical OXC devices have been used for this purpose. These devices are constructed of a material, such as lithium niobate, generally in a planar waveguide configuration that allows switching action to take place between various input and output ports. These switching devices do not add a latency or delay to the optical data. However, integrated optical devices have several drawbacks: they are relatively expensive; their minimum size is limited by the physics of optical waveguides; their operation depends strongly on wavelength and is sensitive to polarization; and they result in considerable cross talk and signal attenuation in the fiber optic paths.
In contrast, optical switches based on emerging MEMS technology, including micromechanical or micromachined systems, boast considerable promise for overcoming many of the limitations associated with alternative prior art fiber-optic switching technologies. Optical MEMS systems, also referred to as microoptoelectromechanical systems (MOEMS), use microoptical elements that reflect, diffract, refract, collimate, absorb, attenuate, or otherwise alter or modulate the properties and/or path of a light beam or signal. These types of optical switches can be made very compact and small, typically within the micrometer to millimeter range. The insertion loss of a MOEMS switch interface is comparable to alternative technologies, and occurs mainly at the entry port of the switch where light leaves a first optical fiber and at the exit port of the switch where light re-enters a second optical fiber. These losses are due to the enlargement of the beam dimensions in free space, however, as will be appreciated by those skilled in the art, using appropriate lenses can decrease this effect. The medium of a MOEMS switch is typically air, but a vacuum, inert gas, or other suitable fluid may also be used. The transmission of light within the switch medium amounts for only a small portion of the overall attenuation. Additionally, the non-blocking medium of the switch ensures that no interference occurs when different light paths cross, enabling light beams to traverse without mutual effect, attenuation, or cross-talk: see generally, Hecht J., xe2x80x9cOptical switching promises cure for telecommunications logjamxe2x80x9d, Laser Focus World, page 69, (September 1998), the contents of which are incorporated herein by virtue of this reference. This property further enables the utilization of MOEMS switches in complex array configurations.
For example, micromachined optical switches often use small mirrors that move to perform a switching operation. By actuating the moving element between a first position in which a light beam is allowed to pass unaffected by the mirror and a second mirror position in which the moving element reflects or interferes with the light beam, the path of an input light beam can be redirected into different outputs or otherwise interfered with. The use of mirrors, in particular, is advantageous since they operate independently of wavelength when reflecting an optical beam. However, MEMS switches or valves may also use other types of moving elements such as attenuators, filters, lenses, collimators, modulators, and absorbers to perform a desired switching operation.
In general, to achieve low attenuation losses in a micromachined optical switch, a mirror or other optical element should be very smooth and of optical grade. In addition, the principle and means used to actuate the moving element of a MEMS device should be fast, simple, and provide reproducible and accurate alignment of the moving element. Furthermore, the actuator must be able to move that element by a sufficient amount to accomplish the switching task.
Several prior art MEMS optical switching devices are known,. as for example those described by Toshiyoshi et al., xe2x80x9cElectrostatic Micro Torsion Mirrors for an Optical Switch Matrixxe2x80x9d, Journal of Microelectromechanical Systems, vol. 5 no. 4, p. 231 (December 1996) and by Marxer et al., xe2x80x9cVertical Mirrors Fabricated by Deep Reactive Ion Etching for Fiber Optic Switching Applicationsxe2x80x9d, Journal of Microelectromechanical Systems, vol. 6, no. 3, p. 277 (September 1997). Aksyuk et al. in U.S. Pat. Nos. 5,923,798, 5,943,454, and 5,995,688 also disclose several embodiments of a MEMS optical switching device having an actuator that is mechanically linked to an optical interrupter such as a modulator or mirror. The actuator is provided on a support substrate, and the optical interrupter is vertically or perpendicularly disposed to the surface of the substrate. The actuator, which includes a moveable and a fixed electrode, imparts a motion to a mechanical linkage that in turn causes the interrupter to move within the vertical plane, and thereby into or out of the path of an optical signal. Jerman et al. in U.S. Pat. No. 5,998,906 discloses an electrostatic microactuator having first and second electrode comb drive assemblies, one fixed to a substrate and the other moveable thereupon. A mirror aligned perpendicularly to the surface of the substrate is actuated between a retracted and an extended position to selectively provide an optical switching function. Similarly, Riza et al. in U.S. Pat. No. 5,208,880 discloses an optical microdynamical switch having a mirror securely and mechanically coupled to a piezoelectric actuator which, in turn, is disposed on a substrate. The mirror is oriented perpendicularly to the substrate and at an angle of 45xc2x0 to incident light. By translating the mirror, reflected light is selectively directed into a desired output port.
These and other prior art MEMS actuated devices suffer from certain drawbacks. Notably, the optical moving element or mirror of these MEMS switching devices is positioned vertically or perpendicularly with respect to the substrate surface, typically by etching into a wafer or substrate. With such a configuration, during operation of the device, the position of the optical moving element is subject to deviations from the desired normal angle of 90xc2x0, resulting in additional losses being inserted within the system as well as a possible reduction in accuracy and/or repeatability. Also, in many of these designs, the horizontal translation of a vertically positioned mirror (or other generally planar optical moving element) may be considerably slowed by air resistance against the surface of the mirror.
U.S. Pat. No. 5,774,604 to McDonald discloses a reflective micromechanical structure positioned on the support surface of a well, between an input fiber and at least two output fibers. If the structure is in an unaddressed state, parallel to the support surface, light travels unaffected from the input fiber into an in-line output fiber. If the structure is in an addressed state, tilted and at an angle to the support surface, the light is reflected and eventually provided to another output fiber. The state of the structure is controlled by actuating circuitry in the support surface. Again, the insertion loss, repeatability, and accuracy of the McDonald switching device may also be affected by deviations of the desired angle of the structure, particularly since the tilt angle changes with every switching operation. Furthermore, the tilting switching device described by McDonald is only suitable for optical switches having a single input.
More generally, Dhuler et al. in U.S. Pat. No. 5,962,949 disclose a MEMS micro-positioning device designed to precisely position objects during micro-assembly, manipulation of microbiological specimens, or alignment of an optical fiber with another optical element. The device includes a reference surface/substrate, a support fixed to the surface, and a stage. The object, e.g. a fiber, that is to be manipulated or aligned is placed on the stage, preferably in a notch or other receptacle. The stage is suspended above the reference surface, and the support is disposed adjacent to at least one and preferably two sides of the stage by means of springs. First and second actuators on the support are used to move the stage, and objects carried by the stage, in perpendicular directions within a horizontal plane. The actuators include a number of thermally activated arched beams that are connected to an actuator member that extends toward the stage. When the beams are heated, they expand toward the stage causing the actuator member to push the stage in a fixed direction. One or more vertical actuators are used to bend the stage, and thereby move the specific portion of the surface of the stage on which the object is located in a desired vertical direction. Due to the nature, shape, and bending of the stage, the MEMS actuator disclosed by Dhuler et al. is not suitable for precisely holding a generally flat or planar shaped element such as a mirror. Furthermore, the actuator is only capable of moving the stage within a small range of travel for alignment purposes. This is insufficient to accommodate a moving element that must be actuated along a relatively long travel path, as for example in an optical switch where the element is selectively actuated out of and into the path of an optical signal. Consequently, the MEMS actuator disclosed by Dhuler et al. is inappropriate for use as an optical switch that actuates a moving element such as a mirror. Other prior art MEMS device actuators, such as the comb drive actuator described by Ye et al. in xe2x80x9cOptimal Shape Design of an Electrostatic Comb Drive in Microelectromechanical Systemsxe2x80x9d, Journal of Microelectromechanical Systems, vol. 7, no. 1, p. 16 (March 1998) are similarly limited with respect to the permissible range of travel of a moving element connected thereto.
In addition, in prior art MEMS devices that actuate a moving element the design of the actuator and the mechanical coupling of the actuator to the moving element typically generates a significant amount of dynamic friction during actuation: see for example Akiyama et al., xe2x80x9cScratch Drive Actuator with Mechanical Links for Self-Assembly of Three-Dimensional MEMSxe2x80x9d, Journal of Microelectromechanical Systems, vol. 6, no. 1, p. 10 (March 1997). As such devices are operated over time, the dynamic friction tends to wear the device components and reduce the reliability and positioning accuracy of the device. Similarly, the moving element of these MEMS devices are generally attached to the substrate or a support component of the device by means of weights, springs, clamps, or other like mechanisms. Again, because these parts are in physical contact with one another, there is dynamical friction during actuation and the parts may wear, leading to reduced device accuracy.
There is therefore a need for an improved MEMS device capable of rapidly and efficiently actuating a generally flat or planar moving element such as a mirror to provide, for example, a switching operation. It would further be desirable if such a MEMS device were not susceptible to wear from dynamic friction effects and exhibited minimal insertion loss when used as an optical switch or cross connect.
The present invention provides a microelectromechanical (MEMS) device having a generally planar moving element disposed in parallel to the surface of a substrate; and an actuator operatively engageable with the moving element for selectively moving the element between a first position in a plane horizontal to the surface of the substrate and a second position in that plane. The moving element preferably travels in a linear path, but others paths such as radial are also possible.
The device is particularly suitable for use as an optical switch where the moving element alters the characteristics of an optical beam when in the first position but does not affect the optical beam when in the second position. In this case, the moving element preferably comprises a mirror, but it may also comprise a modulator, lens, collimator, attenuator, filter, or absorber. The substrate may include a zone which is penetrable by the optical beam and the optical beam may be directed at the device so that the optical beam passes through the penetrable zone when the moving element is in the second position. The penetrable zone may be an aperture formed within the substrate or it may comprise an optically transparent material.
In one embodiment, the actuator comprises an elastic material having a surface and positioned between the substrate and the moving element. The actuator further includes an elastic wave inducer for generating a traveling elastic wave on the surface of the elastic material. In this manner, the propagation of the elastic wave on the surface serves to move the moving element. The elastic wave inducer may comprise a first substrate electrode, a second substrate electrode, a ground electrode coupled between the moving element and the surface of the elastic material, and circuitry for providing a first AC electric signal between the first substrate electrode and the ground electrode and a second AC electric signal between the second substrate electrode and the ground electrode. The first and second AC electric signals are out of phase with one another so that a traveling elastic wave is generated.
In another embodiment, the actuator comprises a plurality of elongated actuating beams spaced perpendicularly to and along a travel path of the moving element. Each beam extends substantially parallel to the surface of the substrate and has a tip, and a base that is rigidly fixed with respect to the substrate. The actuator further includes a beam actuator that controllably moves the actuating beams so that the beams that are positioned along the portion of the travel path in which the moving element is located intermittently engage the moving element and thereby move the moving element in a desired direction along the travel path. The beams are preferably conductive and the beam actuator preferably comprises, for each actuating beam: a first electrode connected to the substrate and positioned vertically from that actuating beam with respect to the surface of the substrate; a second electrode connected to the substrate and positioned horizontally from the actuating beam with respect to the surface of the substrate; and circuitry for controllably generating a first electric field between the first electrode and the actuating beam to move that actuating beam in a vertical direction with respect to the surface of the substrate, and a second electric field between the second electrode and the actuating beam to move that actuating beam in a horizontal direction with respect to the surface of the substrate.
Where the travel path is linear and has first and second edges, the plurality of actuating beams preferably comprises a first set of actuating beams spaced along the first edge of the travel path; and a second set of actuating beams spaced along the second edge of the travel path, the beam actuator controllably moving the tips of the beams in the first set synchronously with the tips of the beams in the second set. In each of the first and second sets, adjacent ones of the actuating beams that are located along the edge of the portion of the travel path in which the moving element is located may rotate out of phase so that the intermittent engagement of the moving element by adjacent tips in each set is successive. Alternatively, where the moving element rests on static support members fixed to the substrate, in each of the first and second sets, the actuating beams that are located along the edge of the portion of the travel path in which the moving element is located may rotate in phase so that the intermittent engagement of the moving element by said beams in each set is simultaneous.
Other actuators may also be used. In all embodiments, the moving element preferably includes a conductive component, and the device further comprises at least one substrate electrode and circuitry for generating an electric field between the conductive component and the substrate electrode or electrodes to hold the moving element by means of static friction.
The device is preferably fabricated using micromachining techniques, and with the moving element fabricated in a position parallel to the surface of the substrate. More preferably, surface micromachining techniques are employed in which a plurality of material layers are sequentially deposited and etched. Arrays of the devices may also be provided on a common substrate, each device having its own moving element and actuator.